1. Field of the Invention
The present invention relates to a drive apparatus for clamping a tire or an output disc of a bicycle equipped with an electric assist or of various small motor vehicles such as a welfare-specific vehicle at an outer periphery of the tire (or output disc) from both sides of the tire (or output disc) in an axial direction of thereof to drive the tire (or output disc) with a friction force.
2. Description of the Related Art
A drive apparatus employed as electric assist in a bicycle or the like often possesses a mechanism for reducing a rotation speed of a high speed motor with a gear train (or gear units) or a friction-driven planetary roller speed-reduction device to directly drive an axle of wheels or indirectly drive the axle via chains or belts.
Such drive apparatus reduces the rotation speed of the motor (several-thousand revolutions per minute) to or less than twentieth to ninetieth (two-hundred revolutions per minute at maximum) so as to obtain a large torque and drive the wheel axle. In order to achieve a large speed reduction ratio, the speed reduction device has to have a large dimension. Further, the drive apparatus should be rigid to bear great forces imposed on various parts thereof upon speed reduction. This often results in not only an increased weight of the drive apparatus but also additional reinforcement required to a bicycle body for supporting the large forces.
Since the bicycle requires drive power sufficient for its weight increased due to the motor, the speed-reduction device, the battery and the body reinforcement parts, the total weight of the bicycle is further increased. As a result, a common bicycle equipped with an electrically assisting device (drive apparatus) is almost twice as heavy as an ordinary bicycle without the electrically assisting device, and its cost is greatly raised.
In order to avoid such great increase in the weight and cost, portions to be driven should be limited to those having a large diameter such as the tire, rim or nearby spokes. By doing so, a speed-reduction device of large reduction ratio is not needed, the bicycle only requires a small reduction ratio to drive itself, and forces acting on various parts are reduced so that no reinforcement is required to the body. Accordingly, the drive apparatus including the drive mechanism can be designed to be compact and light-weight. A low cost drive apparatus that can be attached to the common existing bicycle is therefore obtainable.
To such an end, some bicycles are provided with a drive apparatus of which driving roller is forced against an outer diameter surface of a tire and rotated so as to drive the wheel with a friction force generated between the driving roller and the tire outer diameter surface.
If the drive apparatus drives the wheel with the friction force of the driving roller in contact with the outer diameter surface of the tire, it is necessary to apply a large presser force having a certain margin in order to prevent a slip from occurring between the drive surface of the driving roller and the outer diameter surface of the tire in spite of weather conditions such as rain that makes the road and tire slippery, adhesion of ground, sand and mud, and mechanical factors such as tire air pressure drop, tire wear, axle eccentricity, vehicle body deformation and wheel deformation. Since a large contact area should be insured to maintain a contact force between the driving roller and the tire, and the biting of the driving roller into the tire should be reduced, the driving roller has to have a relatively large diameter.
However, when the bicycle sticks in the mud, for example, a large amount of mud adhering to the tire outer diameter surface is not easy to spin off during the riding. Thus, a slip occurs between the tire and the driving roller and the tire is locally worn. This can disable further movement of the bicycle.
In order to compensate for the drop in the driving force due to the adhesion of the mud, the driving roller should be pressed against the tire with a still greater force. However, it results in the reduced life of the tire because application of the large force and deformation are repeated.
It is necessary to employ a driving roller of larger diameter and enlarge a contact area between the tire and the driving roller in order to reduce the deformation caused by the large presser force. This inevitably makes the rotation speed of the driving roller slower. In other words, the rotation speed of the motor is greatly reduced and the driving roller is driven by such a motor.
Gears or belts are often utilized to reduce the motor rotation speed considerably. However, if a smaller gear or pulley has a very small diameter, duration drops and therefore extremely small diameter cannot be adopted. Accordingly, a large gear or pulley has to have a large diameter if a reduction ratio is large. This makes it difficult to attach the drive apparatus to the bicycle since a limited space is only available. Consequently, a large reduction ratio cannot be expected, and a low speed motor has to be selected.
In general, a motor can be made compact if it is designed to rotate faster and to demonstrate the same output. Thus, use of a low speed motor is disadvantageous because the size, weight and cost of the motor increase.
The above described drive apparatus that drives the outer diameter surface of the tire therefore has a large size, and is particularly difficult to mount on a bicycle since the installation space for the drive apparatus is limited.
If the tire is held between the two drive rollers that are rotated in opposite directions to drive lateral surfaces of the tire with friction forces, the number of points of drive becomes two so that the drive force is dispersed. Since the lateral surfaces of the tire are less affected by the mud as compared with the outer diameter surface, the drive roller can have a smaller diameter and the drive motor of higher rotation speed can be employed as compared with an arrangement that drives the tire at the outer diameter surface. This is advantageous in terms of size reduction of the drive apparatus.
A common approach to rotate the two driving rollers in opposite directions relative to each other is as follows: two spur gears having the same number of teeth are coaxially provided on the two driving rollers respectively such that they mesh with each other, and one of the gears is driven by a motor. Therefore, each of these spur gears has a pitch circle diameter which has the same value as a distance between axes of the two driving rollers. Thus, the spur gear has a larger diameter than the driving roller and a casing for housing these gears becomes large.
If the two spur gears have a smaller diameter and two intermediate (or loading) gears are located between the spur gears, the size of the casing may be reduced. However, a cost will be raised since bearings for supporting two shafts of the intermediate gears are additionally provided. Further, a larger load is exerted on the teeth as the gear diameter is reduced. Gears used in an inexpensive mechanism such as the electric drive apparatus attached to the bicycle are generally made from plastic. Therefore, the smaller gears may jeopardize strength reliability; use of the small gears is not practical. In sum, a compact gear casing cannot be realized so that the drive apparatus cannot be mounted on the bicycle, and drive power loss is increased due to the increased load acting on the tooth surfaces and the increased points of gear engagement from one to three, and noises are also increased for the same reason.
Further, the drive mechanism employing the plastic gears cannot have sufficient accuracy in the gears and the teeth greatly deform elastically so that increased noises and torque irregularity upon gear meshing are unavoidable.
Moreover, when the bicycle equipped with the electrical drive apparatus runs by its inertia without being assisted by the associated motor, the motor becomes a brake to hinder the bicycle""s coasting as the motor is driven by the wheel. When the bicycle is pushed by hands, the wheel drives the motor so that the bicycle becomes heavy to push. In order to eliminate these inconveniences, one approach may be provision of a one-way clutch between the motor and the drive apparatus. This, however, makes the structure complicated and augments the size, weight and cost.
As described above, the drive apparatus according to the prior art that friction-drives the outer diameter surface of the tire with a friction force between the driving rollers and the tire requires the large diameter driving rollers and the low speed motor to prevent slip between the tire and driving rollers and reduction of life of the tire due to excessive deformations. Thus, the drive apparatus has to have a large dimension, heavy weight and increased cost, and is difficult to mount on the bicycle.
If the drive apparatus is designed to clamp the output disc such as a tire by the two driving rollers and drive the tire with a friction force, and the two driving rollers are adapted to rotate in opposite directions upon engagement of the two spur gears coaxially attached to the driving rollers, then the gears inevitably generate noises and have a large diameter so that the casing for the gears becomes large, the drive apparatus becomes heavy, the cost is raised and the installation onto the bicycle becomes difficult.
Even if the two spur gears have a smaller diameter, and two idler gears are located between the two spur gears, bearings for supporting shafts of these idler gears must be provided. In this case, the distance between the gear centers cannot be reduced since the outer diameters of the bearing interfere with each other. Further, the gears cannot have a small diameter since they should have sufficient strength. Therefore, the size reduction of the gear casing is limited, the weight and cost are raised, and the installation onto the bicycle is hardly improved.
A primary object of the present invention is to provide a drive apparatus that can eliminate the above described problems, i.e., the drive apparatus that has a smaller casing, a reduced weight, is easy to mount on a bicycle, and has a reduced cost.
In the arrangement adapted to cause the two driving rollers to rotate in opposite directions upon engagement of the associated gears, the slip friction loss due to the gear engagement results in deterioration of the power transfer efficiency. This is particularly significant when the inexpensive plastic gears are used. A second object of the present invention is to provide a drive apparatus that can reduce the friction loss caused during power transfer, and has a high power transfer efficiency.
A creep ratio of a tooth surface upon gear engagement is as large as 10% to 20%. The lubricating oil is repeatedly subjected to shearing because of this creep so that the lubricating oil is eventually deteriorated. It is therefore necessary to sufficiently supply a lubricating oil to the gears to reduce frictions and prevent gear surface wear. Further, a sufficient amount of lubricating oil should be sealed inside the gear casing to reduce the influence of deteriorated lubricating oil.
As a large quantity of lubricating oil is sealed, the volume of the air inside the casing decreases. When the casing temperature rises due to heat generation of the gears, atmosphere temperature rise and sunshine, thermal expansion of the lubricating oil and air raises the pressure in the casing and troubles such as oil leakage tend to happen. A third object of the present invention is to reduce an amount of lubricating oil used in the drive apparatus so as to eliminate drawbacks such as oil leakage.
The arrangement that has gears generates noises upon engagement of the gears. Particularly, high accuracy cannot be expected in the plastic gears of low price. Even if a vibration damping effect of the plastic is taken into account, a problem of noises still remains. A fourth object of the present invention is to reduce noises upon drive power transfer.
The arrangement having the gears cannot have small gears since the strength of the gears and the dimensions of the bearings provided to support the gears impose limitations. Thus, a small motor that is able to rotate at a high speed cannot be employed. Accordingly, the motor size reduction, weight reduction and cost reduction are difficult. A fifth object of the present invention is to provide a light-weight, inexpensive, highly reliable and durable drive apparatus that does not suffer from limitations imposed by the gear dimension and can use a small, light-weight and high speed motor.
A sixth object of the present invention is to eliminate necessity of a one-way clutch that prevents bicycle""s coasting from being reduced due to the wheel""s driving the motor during the coasting or bicycle hand-pushing and that prevents the bicycle from becoming heavy to push, and to provide a drive apparatus itself with a function of a one-way clutch.
In case of a welfare-specific vehicle such as an electrically driven wheel chair, on the contrary, the function of the one-way clutch is not needed; both forward and backward movements should be made with the electrical device. A seventh object of the present invention is to cope with such a demand, i.e., to transmit the torque in both directions without adversely influencing other functions.
The present invention is developed in view of the above described background, and its object is to provide a parallel dual-shaft drive apparatus that employs a high speed, small and high output motor, generates less noises, has a high power transfer efficiency, possesses a function of a one-way clutch, which is eliminated upon a demand to enable torque transmission in both directions, has high reliability, is small, has a reduced weight, is easy to manufacture, is easy to mount on a vehicle, is easy to maintain, is inexpensive, and can be utilized as an electrical assisting device to drive lateral surfaces of a tire of a bicycle with a friction force when mounted on the bicycle.
According to one aspect of the present invention, there is provided a parallel dual-shaft drive apparatus comprising: a single casing; a single input shaft supported by bearings in the casing and having an axis X and an input roller, an outer peripheral surface of the input roller being a substantially cylindrical rolling surface; first and second output shafts supported by bearings in the same casing and having axes Y1 and Y2 and first and second output rollers respectively such that the output shafts and input shaft extend in a substantially same plane, the second output roller is positioned adjacent to the input roller, an outer peripheral surface of each output roller is defined by a substantially cylindrical rolling surface, and the rolling surface of the second output roller is in rolling-contact with the rolling surface of the input roller; and a loading roller extending between the input roller and the first output roller such that a center axis Z of the loading roller is slightly spaced from a plane including the axes Y1 and Y2 of the first and second output shafts and extends in parallel to the input shaft axis X and the output shaft axes Y1 and Y2, an outer peripheral surface of the loading roller is defined by a substantially cylindrical rolling surface, the rolling surface of the loading roller is in rolling-contact with the rolling surface of the input roller and the rolling surface of the first output roller, and the loading roller is supported by support means inside the casing and preloaded by a preload spring which causes the loading roller to squeeze between the rolling surface of the input roller and the rolling surface of the first output roller.
As the input shaft rotates, the second output roller and the loading roller in contact with the input roller are caused to rotate in a direction opposite the input roller, and the first output roller in contact with the loading roller is caused to rotate in the same direction as the input roller, i.e., in a direction opposite the second output roller. In this situation, if a load torque is applied to hinder the first output roller from rotating, a tangential force corresponding to the torque acts between the rolling surface of the first output roller and the rolling surface of the loading roller. This tangential force creates a similar tangential force between the rolling surface of the input roller and the rolling surface of the loading roller. Then, a friction force acting on the loading roller caused by the input roller driving the loading roller and a friction force acting on the loading roller caused by the rolling roller driving the first output roller cause the rolling roller to be pulled in between the input roller and the first output roller.
In order to transmit rotations of the rolling surfaces by friction between the rolling surfaces, a normal force that can create a sufficient friction force to prevent slip between the rolling surfaces is needed. In this drive apparatus, a contact point between the rolling surface of the input roller and that of the loading roller and a contact point between the rolling surface of the loading roller and that of the first output roller are situated on the rolling surface of the loading roller at positions slightly deviated from a diameter line of the loading roller so that when a friction force acting between the rolling surface of the input roller and that of the loading roller and a friction force acting between the rolling surface of the loading roller and that of the first output roller function in a direction that causes the loading roller to be pulled in between the input roller and the first output roller, the friction forces are augmented by wedge effect whereby large normal forces sufficient to prevent slip at these contact points are generated.
The normal force augmented by this wedge effect causes a slight elastic deformation to occur in a normal direction relative to the rolling surface at the respective contact point between the rolling surfaces of the rollers, and the loading roller is slightly shifted such that it squeezes between the input roller and the first output roller and the center axis Z thereof is shifted slightly towards the plane including the axes Y1 and Y2 of the first and second output rollers.
The large normal force applied between the rolling surface of the input roller and that of the loading roller is also applied between the rolling surface of the input roller and that of the second output roller via the input roller. Thus, it is also possible to transmit the rotations by a friction force without causing slip between the rolling surface of the input roller and that of the second output roller. As a result, the driving rollers mounted on the first and second output shafts can friction-drive the tire simultaneously.
At this point, if the input roller stops and the first output roller keeps rotating in the same direction, then the friction force acting between the rolling surface of the first output roller and that of the loading roller reverses its direction and functions to push out the loading roller between the input roller and the first output roller. Accordingly, no wedge effect is produced and large normal force is no longer generated so that slip occurs between the rolling surface of the first output roller and that of the loading roller. Losing the large normal force owing to the wedge effect results in losing the normal force acting between the rolling surface of the input roller and that of the second output roller so that slip also occurs there.
Specifically, when rotations should be transmitted from the input shaft in a certain direction, the wedge effect of the loading roller produces a large normal force between the rolling surfaces to apply a friction force so that it is possible to transmit rotations to the first and second output rollers. When trying to transmit rotations of the same direction from the output shaft, however, the friction force reverses its direction and acts in a direction to push out the loading roller from between the input roller and the first output roller. Consequently, the wedge effect is lost, and a function of a one-way clutch that prohibits transfer of rotations from the first and second output rollers to the input shaft is obtained.
Since the drive apparatus of the invention transmits the drive power by friction, large normal force caused by the wedge effect acts between the rolling surfaces of the input roller, first and second output rollers and loading rollers. Therefore, these rolling surfaces are generally formed from a hard metal that undergoes quenching. A lubricating oil is required to the friction surface in order to enable rolling contact between the hard metal surfaces.
Use of a traction oil that has a traction coefficient, which is a friction coefficient several times as much as a common lubricating oil, is indispensable to transmit a large friction force while rolling. The traction oil demonstrates its high traction coefficient when it is used under a high contact pressure such as 1 GPa or more, which is a value higher by approximately two orders of magnitude when compared with the material (i.e., plastic) of the plastic gear. The highest traction coefficient is obtained when the contact pressure is 1.5 to 2 GPa. This value is substantially the same level as a contact pressure between a bearing ball and a bearing casing under an ordinary operating condition. This value is a reasonable value, which has been sufficiently examined with respect to the quenched bearing steel, and the bearing steel can demonstrate a practically sufficient rolling fatigue life under such operating condition and the life can be predicted to a certain extent.
When the drive apparatus is utilized for a bicycle to electrically assist movements of the bicycle, the input roller, output rollers and loading roller should have a considerably small size to meet the above mentioned high contact pressure condition. Therefore, it is possible to design this drive apparatus to be significantly compact and have greatly elongated life as compared with a conventional drive apparatus including plastic gears. Thus, the size reduction, weight reduction and cost reduction as well as high reliability are all promised.
In order to effectively use the high traction coefficient property with the shearing resistance of the traction oil without causing a large slip when the cylindrical surfaces are in rolling-contact with each other like in this drive apparatus, a ratio of a surface speed difference between the two rolling surfaces to the surface speed, i.e., a creep ratio, being 0.5% or less (normally a creep ratio of about 0.3 to 0.4%) is the most appropriate value. There is no substantial transfer loss at the rolling surface except for this slip. In other words, the transfer loss at the rolling contact between the cylindrical surfaces is 0.5% or less, and is sufficiently smaller than the transfer loss in case of gear-to-gear transfer. Thus, a high power transfer efficiency is obtained.
In this drive apparatus, the bearings supporting the input roller bears little load, and a load acting on the bearing supporting the output roller that supports a large load exerted on the rolling surface is mostly a radial load except for an axial load derived from a small component directed in an axial direction due to a cone angle of a drive surface of the driving roller. A friction torque of the radial ball bearing is extremely smaller as compared with a case where an axial load is exerted. Therefore, a friction loss due to the bearing is small. For this reason also, a high power transfer efficiency is obtained.
In the drive apparatus, the rolling surfaces and other parts/portions adapted to transfer drive power have simple cylindrical surfaces or similar configurations so that they can be machined at high precision and low cost like the roller bearings.
Since the creep ratio is small at the rolling surface which requires a lubricating oil so that heat generation is small and deterioration of the lubricating oil is suppressed. As a result, it is sufficient to supply a trace amount of lubricating oil to the rolling surface. Enough lubricating oil feeding is achieved by simply placing a pad impregnated with the lubricating oil on the rolling surface. The lubricating oil is ensured to have a long life.
Further, since no intermittent contact like the gear meshing does not occur, noises generated by the rolling motion of the cylindrical surfaces are very small, not comparable with noises caused by the gears.
Moreover, the traction oil is confirmed to exhibit a high traction coefficient to a high speed rolling over 10 m/sec so that no problems would arise even if the input roller is caused to rotate at a high speed over 10,000 rpm. In the drive apparatus, the diameter of the input roller""s rolling surface is smaller than that of the output roller""s rolling surface so that the rotation speed of the input roller can be raised as compared with a case where the output roller is directly driven by the motor. Therefore, a compact, light-weight and high speed motor can be used, and the drive apparatus that is compact, light-weight, easy to mount on a vehicle, has a high power transfer efficiency and is inexpensive is obtained.
The input shaft X of the drive apparatus may extend in the substantially same plane as a plane including the axis Y1 of the first output shaft and the center axis Z of the loading roller.
During operation, friction forces act on the rolling surface of the input roller at a contact point between the input roller""s rolling surface and the second output roller""s rolling surface and at a contact point between the input roller""s rolling surface and the loading roller""s rolling surface. If the torque applied to the first output roller is equal to that applied to the second output roller, the friction forces have the same magnitude and the same normal forces act on the two contact points. In this case, the axis X of the input shaft extends in substantially the same plane as the plane including the axis Y2 of the second output shaft and the center line Z of the loading roller so that their normal forces are situated at symmetrical positions on the rolling surface of the input roller along the diameter line of the input roller and counterbalanced each other in principle. Thus, no radial load is exerted on the bearings of the input shaft. Friction forces acting on the input roller are also situated at symmetrical positions on the rolling surface of the input roller along the diameter line and have the same magnitude in opposite directions so that they form a couple. This further proves that no load is exerted on the bearings that support the input shaft.
In actuality, however, a normal force generated at a contact point upon application of a torque causes a mutual approach at the rolling surface and/or bearings so that a position of the center line Z of the loading roller is slightly shifted towards the plane including the axis Y1 of the first output shaft and the axis Y2 of the second output shaft. In addition, it cannot be said that the normal forces and friction forces acting on the two contact points are always exactly the same. A small radial load is therefore exerted on the bearings supporting the input shaft.
If the load exerted on the input shaft has a small value, the rigidity of the input shaft support may be lower than the output shaft so that an overhang structure can be adopted to support the input shaft. Accordingly, a motor shaft itself, which is inserted into the casing from the outside to drive the input shaft, can be used as the input shaft. This contributes to simplification of the drive apparatus structure and cost reduction.
Every other rolling surface of the first output roller, loading roller, input roller and second output roller may have a cylindrical surface, and the remainder may have a crowned cylindrical surface.
When two cylindrical surfaces having parallel axes contact each other and make a line contact, significantly large contact pressures, i.e., edge loads, appear in the vicinity of longitudinal ends of a contact surface. The edge loads promote rolling fatigue of the rolling surface and greatly reduce the life. To prevent the edge load, the cylindrical surface has to have a rounded surface with a large radius of curvature in the axial direction, like the roller bearing. Specifically, the crowning process may be applied to the whole cylindrical surface such that the line contact is changed to a point contact. Alternatively, the crowing may be applied to ends of the cylindrical surfaces such that the contact pressure in the contact area is regulated as much as possible.
By applying such crowing to the rolling surface, it is feasible to prevent the life of the drive apparatus from being reduced significantly from an expected duration.
In general, only one of the two contacting cylindrical surfaces needs to undergo the crowing process to avoid the edge load. In case of four rolling surfaces, therefore, the crowing process is applied to the two non-neighboring rolling surfaces. This contributes to cost reduction when compared with a case where the crowning process is applied to the four rolling surfaces.
A diameter of the rolling surface of the first output roller may be slightly smaller than a diameter of the rolling surface of the second output roller.
When a traction drive is employed, in which the drive power is transferred by a shearing resistance of an oil present between two rolling surfaces, slight creep exists between the rolling surfaces. In order to stably transfer the drive power, the traction oil should be utilized in a so-called proportional zone, in which the traction force increases substantially in proportion to the increasing creep ratio. The creep ratio in this range is approximately 0.5% or less as mentioned earlier.
When the creep exists even if small, and the same torque is applied to the first output roller and the second output roller, the first output roller that transfer the drive power via two contact points in turn will have a larger (summed up) creep ratio than the second output roller that transfers the drive power via a single contact point. Thus, the rotating speed of the first output roller is reduced correspondingly. In other words, if the rotating speed of the first output roller is set to be equal to that of the second output roller under a non-creep condition, a torque applied to the second output roller that is subjected to a smaller creep becomes greater than a torque applied to the first output roller that is subjected to a large creep.
In practice, when the two driving rollers mounted on the first and second output rollers friction-drive the lateral surfaces of the tire, the difference in the torque applied to the two output rollers is more moderated than the creep ratio difference by the elastic deformations at the contact areas between the driving rollers and the rubber tire, but the torque difference unavoidably increases with the increasing torque.
In order to reduce the transferred torque difference between the two output rollers due to the difference in the number of the contact points which the torque passes during transfer, a magnitude of a torque that should transfer the drive power at the maximum efficiency is first determined, and the rotation speed of the first output roller under a non-load condition is set to be slightly faster than that of the second output roller by an amount corresponding to the creep ratio difference, such that the same torque is applied to the two driving rollers when the above determined torque is reached.
By slightly reducing the diameter of the rolling surface of the first output roller than that of the second output roller in correspondence with the creep ratio, the difference in torque applied to the two output rollers is reduced.
Two loading rollers may be provided between the input shaft and the first output shaft such that they may be arranged in tandem, and center axes of these loading rollers may be spaced in opposite directions from the plane including the axis Y1 of the first output shaft and the axis Y2 of the second output shaft and may extend in parallel to the input and first and second output shafts.
A welfare-specific vehicle such as an electrically driven wheel chair should be able to move forward and backward. Thus, the function of the one-way is unnecessary. In order to apply an appropriate normal force between the input roller or output roll and the loading rollers on one hand and to transfer a torque to cause rotations of forward and backward directions on the other hand, the two loading rollers are needed to work in relation to the forward rotation torque and the backward rotation torque respectively.
This function is realized by arranging the two loading rollers on the opposite sides of the plane including the axes Y1 and Y2 of the two output shafts such that the two loading rollers produce the wedge effect with the oppositely directed torques.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.